Join Date: Oct 2007
Location: Québec, Québec, Canada
Bikes: Fuji Touring 64cm, Peugeot Canada PB-18 61cm and winter beaters
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I understand that while racing, you downshifted from the big front chainring to the small front chainring. I imagine that you were applying racing power at that time, and that the chain was thrown to the inside, further inwards than the small chainring.
Well, this might not be something you can solve completely with a simple derailleur adjustment... But still, it's worth a try. What you want to adjust is the front derailleur low limit screw. Shift to the small front chainring and to the big rear cog. Look at the cage of the front derailleur - there should be a very small clearance between the cage and the chain. If the cage is too far away from the chain, you can screw in (clockwise) the low limit screw to bring it closer.
You'll also need to pick up some slack in the cable by unscrewing (counterclockwise) your front derailleur control cable barrel adjuster.
Now, this may not solve your problem. In fact, your front derailleur might already be adjusted correctly. Suicide downshifts such as you have experienced do happen when shifting quickly while applying lots of power.
There is a permanent solution, it's called a chain guide, or anti-derailment device. The N-Gear Jump Stop is one of them. Such a device will guide the chain back onto the small front chainring if it should be thrown too far inside.